1. Field of the Invention
This invention pertains generally to a voltage controlled oscillator (VCO), and more particularly to a VCO configured for compensating for device process variations.
2. Description of Related Art
A voltage controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input, with the applied input voltage determining the instantaneous oscillation frequency. Thus, modulating signals applied to the control input can be utilized to create frequency modulation (FM) or phase modulation (PM), or as part of a phase-locked loop, or in a wide range of other applications.
A VCO consisting of a chain of digital inverter rings is considered to have higher jitter compared with VCO with L-C tank circuits. Despite these shortcomings, the ring-type VCO is widely used in integrated circuits, due to its much smaller chip size as a logic circuit. Typical ring-type VCOs utilize a diode to limit the voltage swing toward achieving some degree of control over operation variation. However, even the characteristics of these diodes vary as fabrication processes change.
Accordingly, a need exists for a VCO which provides low jitter by overcoming fabrication process variations.